I think one exists by default called Incident changed, which you could just enable.  But if you don't have a rule that works already for you out-of-the-box, follow these steps to create a Notification Rule. 

This creates a notification rule that:

• sends an e-mail to the assigned worker when the incident changes (but not on creation, because that's covered by incident creation and incident assignment notifications)
• when it's assigned to a worker (but not when it's reassigned, because that's covered with an Incident reassignment notification rule)
• does not send an e-mail if the change was made by the assigned worker (the assigned worker already knows -- he or she made the change!)

Create a notification rule

1. Open your Helpdesk Worker Console (as an admin)
2. Under Commands, Choose Admin->Notify Rules->New Notify Rule
3. Give the rule a name (Incident change notification) and description (Sends an e-mail to the assigned worker when an incident is changed by a guest or worker other than the assigned worker.)
4. In the Send e-mail drop-down box, choose Change notification
5. For the Send To option, place a checkmark in the To box for the Assigned worker
6. Check the box that says 'Don't send mail to worker editing the incident'
7. Under the When, change to 'Only when the incident already exists'
8. Under And, click <none specified>
9. Click the drop-down that says Action by default
10. Choose Version
11. Click Add
12. Check the box for "When the value of 'Version' changes"
13. Click OK
14. With "Version" changes highlighted, click the {} button
15. With "Version" changes highlighted, click the blue "Up" arrow
16. Click <none specified>
17. Click the drop-down box that says Action by default
18. Choose Assigned Worker
19. Click Add
20. Check the box for "When the value of 'Assigned Worker' changes"
21. Check the box for When the value of 'Assigned Worker':" and change '[equal to]' to '[is not equal to]', leaving the second box as [unassigned]
22. Click OK
23. Click on the When ALL of these are TRUE that is in between Version and Assigned Worker -- that is, the second one
24. Click the edit pencil
25. Change to 'When ANY of these is FALSE' and click OK
26. Your statement now reads:
    When: Only when incident already exists
    And:
      When ALL of these are TRUE
          "Version" changes
          When ANY of these is FALSE
                "Assigned worker" changes and is not equal to "[unassigned]"
          <end>
      <end>
27. I recommend checking Locked, Default, and Active, and unchecking Visible -- this makes it invisible when viewing an incident, default so that it is enabled on every incident, active so that it runs, and locked so that workers cannot change it
28. Click OK to save your rule

Does this solve your problem?